Greek banks saw their share prices swing sharply on Friday after the country's central bank said they need to raise billions of euros to plug holes in their finances.

Late Thursday, the Bank of Greece said the sector needs to raise 6.4 billion euros to be able to cushion potential future losses, an announcement that prompted some of the banks to outline their plans.

The most noteworthy mover was Piraeus Bank, Greece's biggest bank by market capitalization. Though its share price was down 2 percent, it had earlier fallen as much as 7 percent after the bank said it plans to raise 1.75 billion euros in a share capital increase. The prospect of more shares in circulation tends to weigh on the share price.

Though Piraeus's needs were estimated at 425 million euros, the bank said it is taking advantage of an improved market backdrop to raise more, adding that it aims to repay in full a 750 million-euro government investment.

The bank said the share capital increase will make it "strong enough to withstand extreme economic conditions and to decisively support the economic progress of the country."

The fact that Piraeus is confident enough to raise more money than it is required to do is a further sign of improvement in Greece's financial markets as the country's public finances heal gradually and the economy is expected return to growth soon.

The ATHEX stock market in Athens is trading near three-year highs and the government's borrowing rates in bond markets have fallen sharply in recent months. The ATHEX general index expanded 1.68 percent on Friday to end at 1,341.24 points, posting weekly gains of 2.35 percent.

Greece has been hammered by a vicious financial crisis since late 2009 that developed into an economic depression. The economy has shrunk by around a quarter while unemployment has soared to over 25 percent.

Since May 2010, Greece has been dependent on billions of euros in rescue loans from the other European Union countries that use the euro, and from the International Monetary Fund. In return, successive Greek governments have had to slash spending, increase taxes and enact wide-ranging economic reforms.

Piraeus wasn't the only bank in focus on Friday.

Eurobank said it was asking shareholders to give it the right to raise the share capital to "fully cover" the 2.95 billion euros the Bank of Greece thinks it needs. Its shares also fell, but recovered somewhat through the day, to trade 4 percent lower at 0.41 euros.

Shares in the National Bank of Greece were roughly flat after it said it won't raise money to meet the 2.2 billion-euro hole estimated by the central bank. It said it will present a plan in April that "will address the capital needs without raising new equity capital."